Field of the Disclosure
The present invention relates generally to power supplies, and more specifically, the invention relates to switch mode power supplies.
Background
Many electronic devices, such as cell phones, laptops, etc., are powered by a source of direct current (dc) power. Conventional wall outlets generally deliver a high voltage alternating current (ac) power that needs to be transformed to dc power in order to be used as a power source by most consumer electronic devices. Switch mode power converters, also referred to as switch mode power supplies, are commonly used due to their high efficiency, small size, and low weight to convert the high voltage ac power to a regulated dc power. In one example, switch mode power converters are used to provide regulated power to light emitting diode (LED) devices.
One important consideration for a switch mode power converter is the shape and the phase of the input current drawn from the power source relative to the ac input voltage. The shape of the ac input voltage is typically sinusoidal but because a switching power converter presents itself as a non-linear load, the shape of the input current drawn from the power source may become distorted (non-sinusoidal) and/or out of phase with ac input voltage. This results in increased power loss in the power distribution systems.
Correction of the input current waveform to reduce shape and/or phase mismatch with respect to input voltage is referred to as power factor correction (PFC). The power factor may be defined as the ratio of the average power over a cycle to the product of the root mean square (rms) voltage and the rms current. That is, the power factor may represent the ratio of the amount of usable power to the amount of total power delivered to the load. As such, the power factor may have a value between zero and one, with unity power factor being the optimal. If the input current is sinusoidal and perfectly in-phase with the input voltage, the power factor of the power supply is one, and none of the energy delivered to the load is returned to the power source. However, as the switch mode power supply distorts the wave shape of the input current and/or introduces a phase shift with respect to the input voltage, the power factor decreases. Several regulatory agencies have set tight standards that typically stipulate for greater power factors and/or lower harmonic content of the input current.
One example application where switch mode power supplies may be required to perform PFC is power conversion systems that are used in light emitting diode (LED) lighting. Since the brightness of light provided by LED lamps is a function of the current through LEDs, the power supply used in such a system may also regulate the current provided to LEDs at the output of the power supply. In other words, the power supply may provide both output current regulation and PFC.
Output current regulation is typically achieved by a power supply controller by sensing the current provided to the LEDs. A feedback signal is used to represent a current through the LEDs. The power supply controller controls the transfer of energy from an input to an output of the power supply in response to the feedback signal. Switch mode power supplies typically respond very quickly to fluctuations in the feedback signal by adjusting the energy transfer to regulate the LED current at a desired level. However, making rapid changes to the energy transfer can compromise the PFC performance and cause the input current to be non-sinusoidal and/or out of phase with the input voltage, resulting in a reduced power factor.
A switch mode power supply may use a controller to control the switching (i.e., the turning on and turning off) of a power switch to provide a desired output to a load. The controller may regulate the output at a desired level in response to a feedback signal representative of the output of the power supply. Some controllers may use a digital control signal to adjust the operating condition (e.g., on-time, switching frequency) of the power switch in response to the feedback signal. Such a controller may employ a digital-to-analog converter (DAC) to convert the binary values of the control signal to corresponding discrete levels of an analog signal that may be used to set the operating condition of the power switch. For some types of DACs, such as binary-weighted DACs, as the number of the bits of the control signal is increased, the number of different operating conditions to which the power switch can be set is increased. As a result, the area on the silicon occupied by the DAC components such as current sources, resistors, etc., may grow and make such an implementation impractical.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.